Ion implanters are commonly used in the production of semiconductor wafers. An ion source is used to create an ion beam, which is then directed toward the wafer. As the ions strike the wafer, they dope a particular region of the wafer. The configuration of doped regions defines their functionality, and through the use of conductive interconnects, these wafers can be transformed into complex circuits.
A block diagram of a representative ion implanter 100 is shown in FIG. 1. An ion source 110 generates ions of a desired species. In some embodiments, these species are atomic ions, which may be best suited for high implant energies. In other embodiments, these species are molecular ions, which may be better suited for low implant energies. These ions are formed into a beam, which then passes through a source filter 120. The source filter is preferably located near the ion source. The ions within the beam are accelerated/decelerated in column 130 to the desired energy level. A mass analyzer magnet 140, having an aperture 145, is used to remove unwanted components from the ion beam, resulting in an ion beam 150 having the desired energy and mass characteristics passing through resolving aperture 145.
In certain embodiments, the ion beam 150 is a spot beam. In this scenario, the ion beam passes through a scanner 160, which can be either an electrostatic or magnetic scanner, which deflects the ion beam 150 to produce a scanned beam 155-157. In certain embodiments, the scanner 160 comprises separated scan plates in communication with a scan generator. The scan generator creates a scan voltage waveform, such as a sine, sawtooth or triangle waveform having amplitude and frequency components, which is applied to the scan plates. In a preferred embodiment, the scanning waveform is typically very close to being a triangle wave (constant slope), so as to leave the scanned beam at every position for nearly the same amount of time. Deviations from the triangle are used to make the beam uniform. The resultant electric field causes the ion beam to diverge as shown in FIG. 1.
In an alternate embodiment, the ion beam 150 is a ribbon beam. In such an embodiment, there is no need for a scanner, so the ribbon beam is already properly shaped.
An angle corrector 170 is adapted to deflect the divergent ion beamlets 155-157 into a set of beamlets having substantially parallel trajectories. Preferably, the angle corrector 170 comprises a magnet coil and magnetic pole pieces that are spaced apart to form a gap, through which the ion beamlets pass. The coil is energized so as to create a magnetic field within the gap, which deflects the ion beamlets in accordance with the strength and direction of the applied magnetic field. The magnetic field is adjusted by varying the current through the magnet coil. Alternatively, other structures, such as parallelizing lenses, can also be utilized to perform this function.
Following the angle corrector 170, the scanned beam is targeted toward the workpiece 175. The workpiece is attached to a workpiece support. The workpiece support provides a variety of degrees of movement.
The workpiece support is used to both hold the wafer in position, and to orient the wafer so as to be properly implanted by the ion beam. To effectively hold the wafer in place, most workpiece supports typically use a circular surface on which the workpiece rests, known as a platen. Often, the platen uses electrostatic force to hold the workpiece in position. By creating a strong electrostatic force on the platen, also known as the electrostatic chuck, the workpiece or wafer can be held in place without any mechanical fastening devices. This minimizes contamination and also improves cycle time, since the wafer does not need to be unfastened after it has been implanted. These chucks typically use one of two types of force to hold the wafer in place: coulombic or Johnson-Rahbeck force.
The workpiece support typically is capable of moving the workpiece in one or more directions. For example, in ion implantation, the ion beam is typically a scanned or ribbon beam, having a width much greater than its height. Assume that the width of the beam is defined as the x axis, the height of the beam is defined as the y axis, and the path of travel of the beam is defined as the z axis. The width of the beam is typically wider than the workpiece, such that the workpiece does not have to be moved in the x direction. However, it is common to move the workpiece along the y axis to expose the entire workpiece to the beam.
Temperature, in particular, the temperature of the substrate onto which a particular ion or species is being implanted, plays an important role in ion implantation. While many ion implants are done at room temperature, there are benefits to performing implantation at other temperatures.
For example, low temperature implantation is known to reduce the number of end of range (EOR) defects. When ions are implanted into a substrate, they penetrate to a certain depth. Within this region, the implanted ions serve to change the normal crystalline structure of the substrate, such as silicon, into an amorphous structure. Those depths of the substrate that are not reached by the ions remain crystalline in structure. Therefore, there exists an interface between these two regions, known as the amorphous/crystalline interface. Near this interface, at the lower portion of the amorphous region, is an area that contains a higher density of interstitials. When the substrate is annealed after implantation to activate the dopant and to recrystallize this region, residual non-homogeneities cause residual defects. These defects are called the end of range (EOR) defects. These defects can take the form of dislocations and stacking faults.
These EOR defects, when present in the source or drain regions, cause junction leakage, which ultimately affects the performance of the final semiconductor component. As noted above, low temperature ion implantation has been shown to reduce the generation of EOR defects, thus improving component performance. This feature is especially important in ultrashallow junctions, where the depth of the source and drain regions is very small.
Alternatively, ion implanting or doping into substrates maintained at elevated temperatures (higher than room temperature) can also have benefits. Amorphization of crystalline materials that occurs with implant can be reduced. This may be preferable in applications where ions are being implanted into epitaxially grown substrates. Amorphization tends to destroy inherent properties of doped epi-substrates. Higher temperature implants are also beneficial when the implant dose is less than amorphization threshold. The overall residual damage in the substrate is reduced when such an implant is performed at elevated temperatures. For such low dose implants, heated implanted can also result in lower sheet resistance because of better dopant activation and reduced damage results in smaller amount of ‘transient diffusion’, which can degrade resistivity.
However, each of these temperature implantation modes also has disadvantages. A method of implantation which maximizes the advantages of each temperature implant, while minimizing the disadvantages would be very beneficial.